1. Field of the Invention
The present invention relates to apparatus and methods for electron beam imaging.
2. Description of the Background Art
The two most common types of electron microscopes available commercially are the scanning electron microscope (SEM) and the transmission electron microscope (TEM). In an SEM, the specimen is scanned with a focused beam of electrons which produce secondary and/or backscattered electrons as the beam hits the specimen. These are detected and typically converted into an image of the surface of the specimen. Specimens in a TEM are examined by passing the electron beam through them, revealing more information of the internal structure of specimens.
Bright field imaging and dark field imaging are often used in the context of TEMs. A bright field image may be formed in a TEM by selecting electrons from a central diffraction spot to form the image. A dark field image may be formed in a TEM by selecting some or all of the (non-central) diffracted electrons to form the image. The selection of electrons may be implemented using an aperture into the back focal plane of the objective lens, thus blocking out most of the diffraction pattern except that which is visible through the aperture.
Dark field imaging is typically less commonly used in SEMs. In SEMs the terminology is used to describe imaging modes yielding contrast sensitive to the surface topography. “Dark field imaging” and “topographical imaging” expressions can therefore be used interchangeably. In general, dark field images are those obtained using electrons emitted from the sample surface at high polar angles and a given range of azimuthal angles. The definitions of the polar angle θ and azimuth angle φ in relation to the scattered electrons emitted from the specimen are shown by illustration in FIG. 1. In FIG. 1, the specimen plane is the x, y plane. The z-axis is normal to the specimen plane. This type of image preferentially shows the sample surface topography by highlighting protrusions and depressions in the surface via shadowing or highlighting. This is analogous to the contrast generated by imaging a surface from an angle as it is illuminated normally by light.
A conventional SEM dark field detection system has a below-the-lens configuration 200 as depicted in FIG. 2. In a below-the-lens configuration 200, so-called “external” or “side” detectors 204 are positioned below the objective lens 202 at the bottom of the electron beam column (near the specimen). Under certain conditions, secondary electrons (SE) emitted at higher polar angles (i.e. closer to the surface), which are generally more sensitive to surface topography, will preferentially reach such below-the-lens detectors 204. Images formed with such detectors show the topography of the surface with an azimuthal perspective defined by the detector positioning with respect to the primary beam optic axis and the sample/wafer plane.
Unfortunately, the below-the-lens configuration is incompatible with final (objective) lens arrangements that immerse the specimen in magnetic and/or electric fields. These fields are needed for minimizing lens aberrations and obtaining the best resolution images, but they interfere with the collection efficiency of below-the-lens detectors 204.
In addition, the polar angle discrimination threshold is not well controlled for such below-the-lens detectors 204 because the electron energy and emission azimuth can affect the polar angle acceptance of the detector 204.
Dark field imaging may also be performed by tilting the sample/wafer plane normal vector away from the primary beam optic axis. Such tilting may be accomplished by tilting the wafer or column and has the effect of changing the angle of incidence of the primary beam. The secondary electron signal detected, either by a side channel detector or a conventional in-lens detector, shows topological DF contrast according to how the primary beam angle interacts with the surface features on the sample/wafer.
Various previous behind-the-lens configurations for an SEM dark field detection system have been employed that are compatible with final lenses with immersion electromagnetic fields at the specimen. These previous configurations distinguish between the different angular components of the secondary-electron emission after it has been captured by the electro-magnetic field and traveled up into the column beyond the final lens.
A typical behind-the-lens configuration 300 for an SEM dark field detection system is depicted in FIG. 3. A typical behind-the-lens configuration 300 uses off-axis detectors 304 similar to those shown in FIG. 3. These may be separated (as shown) or joined together to form a segmented detector. These detectors 304 are located “behind” the objective lens 302. In other words, the detectors 304 are located on the opposite side of the objective lens 302 to the specimen. Various detector geometries and associated electron optics have been used previously to detect scattered electrons with polar angle discrimination. However, behind-the-lens configurations have generally shown inferior dark-field (topographical) contrast compared to below-the-lens implementations.